This invention relates to a starter unit in which the rotation of the armature shaft of a DC motor is transmitted through an epicycle reduction gear to the output shaft, and the rotation of the output shaft is transmitted to an over-running clutch to start the engine.
One example of a conventional starter unit is shown in FIG. 5.
In FIG. 5, reference numeral 1 designates a DC motor with an armature 2 from which an armature shaft 5 is extended. A small gear, namely, a sun gear 5a is formed on the front end portion of the armature shaft 5. The DC motor 1 includes a yoke 6, on the inner cylindrical wall of which field magnets are mounted. The yoke 6 is coupled to a front bracket 8 and a rear bracket 9.
Further in FIG. 5, reference numeral 10 designates an output shaft which is coupled through a steel ball 11 to the armature shaft 5 in such a manner that it is in alignment with the armature shaft 5. More specifically, the front end portion of the armature shafts is engaged, through a bearing 13, with a hole formed in the end face of the rear end face portion of the output shaft 10. The starter unit further comprises: an epicycle reduction gear 15 which is designed as follows: A plurality of planet gears 16 are mounted through bearings 18 on supporting pins 17, respectively, and engaged with the sun gear 5a. The supporting pins 17 are embedded in a carrier 10a which is in the form of a flange formed at the rear end of the output shaft 10. An internal gear frame 19 is secured to the inner cylindrical wall of the front bracket 8. An internal gear 19a a is formed in the inner cylindrical wall of the internal gear frame 19. The internal gear 19a is engaged with the planet gears 16 to revolve around the latter. The rear end portion of the output shaft 10 is supported through a bearing 21 on the inner cylindrical wall of the front end portion of the internal gear frame 19.
The starter unit further comprises: an over-running clutch 22 which is designed as follows: The over-running clutch 22 has a clutch outer member 23 and a clutch inner member 24. The clutch outer member 23 has a helical spline gear 23a formed in its inner cylindrical wall in such a manner that the helical spline gear 23a is engaged with a helical spline gear 10b forme on the output shaft 10. The clutch inner member 24 transmits rotation through rollers 25 to the clutch outer member 23 in one direction, and is mounted through a bearing 29 on the output shaft 10. The front end portion of the clutch inner member 24 is formed into a pinion 26, which is engaged with the ring gear of the engine as the over-running clutch 22 is moved forwardly. Engaging rings 27 and 28 are secured to the clutch outer member 23. A stopper 30 is mounted on the output shaft 10, to regulate the forward position of the over-running clutch 22.
An electromagnetic switch 31 is mounted on the front bracket 8. A hook 33 is fixedly inserted into a plunger 32, which is the movable iron core of the electromagnetic switch 31, in such a manner that the front end portion of the hook protrudes outside of the electromagnetic switch 31. A shift lever 34 in the form of a fork is engaged with the hook 33 and the aforementioned engaging rings 27 and 28. More specifically, the two prongs of the shift lever 34 are engaged with the hook 33, and the base of the two prongs is engaged with the engaging rings 27 and 28 in the axial direction. The shift lever 34 has a protruded middle portion 34 a which is supported on the front bracket 8 in such a manner that the shift lever can be pivoted about the middle portion 34a. An elastic closing member 35 is fitted in a cut formed in the front bracket 8.
The operation of the conventional starter unit thus constructed will be described with reference to the engine on a motor vehicle. When the engine start switch is turned on, the exciting coil (not shown) in the electromagnetic switch 31 is energized to retract the plunger 32 inwardly. The retraction is transmitted, through the hook 33 to the shift lever 34, so that the shift lever 34 is swung counterclockwise in FIG. 5 thereby to move the over-running clutch 22 forwardly. As a result, the pinion 26 is engaged with the ring gear of the engine. At the same time, or when the plunger is retracted as described above, a movable contact (not shown) is moved backwardly to engage with a pair of stationary contacts (not shown) thereby to complete the armature circuit in the DC motor 1. As a result, the armature 2 is rotated. The rotation of the armature 2 is transmitted through the epicycle reduction gear 15 to the output shaft 10. The rotation of the output shaft 10 is transmitted through the over-running clutch 22 to the pinion 26, to rotate the ring gear of the engine.
When the engine is started, the high speed rotation drives the pinion 26 in the same direction; however, it is not transmitted to the armature shaft because the over-running clutch is interposed therebetween.
The above-described conventional starter unit suffers from the following difficulty: If, during the inertial rotation of the engine or starter unit, the start switch is turned on to operate the electromagnetic switch 31, the pinion is engaged with the ring gear again, thus applying an abnormal shock to the starter unit. In this case, the rotation transmitting members may be damaged because the over-running clutch 22 is insufficient in shock absorbing capacity.